Study On Microstructure Evolution And Performance Response Characteristics Of Forged Martensite Stainless Steel Under Extreme Service Conditions

In this thesis,the effect of high-stress cyclic loading at ultra-low temperature of（-180^oC）on the microstructure and mechanical properties at-180^oC of two kinds of martensite precipitation hardening stainless steels（A and B）were studied by tensile and impact tests and microstructure characterization with OM,SEM,TEM and XRD.Effect of cryogenic treatment（holding at liquid nitrogen environment for 1h and then air-cooling）on the microstructure and mechanical properties of unloaded samples were also studied.Results indicate that the cryogenic treatment can refine martensite laths and make a part of residual austenite transfer to martensite.Comparation of mechanical properties tested at room temperature with that at ultra-low temperature show that both A and B steels have significant negative temperature effects.Tensile strengthes（both yield strength and ultimate tensile strength）at ultra-low temperature are increased drastically but the fracture elongation and reduction of area are maintained.The impact toughness is reduced slightly,but with a ductile fracture.Effect of high-stress cyclic loading with four different stress levels(R_p0.2-20MPa,R_p0.2,R_p0.2+20MPa,R_p0.2+50MPa)for 10 cycles and 20 cycles at ultra-low temperature on the microstructure and mechanical properties of A and B steels were studied systematically.Compared with the mechanical properties of unloaded samples,high-stress cyclic loading at ultra-low temperature results in a considerable strain-hardening effect for A and B steels.At ultra-low temperature,the yield strength after high-stress cyclic loading is increased considerably and the plasticity has almost no change.However,its impact toughness is decreased slightly and the fragility is increased a bit.Both the loading stress levels and cycles have almost no influence on the mechanical properties.Dislocation strengthening is demonstrated to be the main cause of the strain-hardening,while in B steel there is weak twin-strengthening.For B steel,the effect of high-stress cyclic loading with a stress level ofζ_0.2 for one cycle and 30 cycles was studied complementally.It is found that just one cycle of loading leads to significant‘strain hardening’with an increment of 108MPa in yield strength for B steel,along with a sharp rise in the dislocation density.But,30 cycles of loading results in a considerable decrease in yield strength compared with 20 cycles,where,the dislocation density is reduced,and the dislocation configurations change obviously and a large area of mechanical twins are formed.This indicates the B steel is softened after 30cycles of loading.